LASORS section D2.
"Pilots who have passed the UK ATPL(A) examinations, who hold a JAR-FCL CPL(A)(R) with Instrument Rating (obtained during the 36 months from the date of gaining a pass in the UK ATPL(A) theoretical knowledge examinations) and a valid type rating on a multi-pilot aeroplane will be required to meet the following requirements for the issue of a JAR-FCL ATPL (A):-
Achieve 1500 hours as pilot of aeroplanes, to include;
500 hours on multi-pilot aeroplanes,
250 hours PIC of which up to 150 hours may be PICUS,
200 hours cross country flight of which 100 hours may be as P2 or PICUS,
75 hours of instrument time of which not more than 30 hours may be instrument ground time,
not more than 100 hours may be in a flight simulator."
The key thing to remember is that you hold a valid JAA licence that was issued on the basis of the national exams. Part A of LASORS actually makes provisions for UK CPL and ATPL holders to convert to a JAA licence.